The invention relates to a mold, and in particular, a mold for encapsulating an electronic device.
Conventionally, electronic devices, such as silicon chips (or dies) are assembled by attaching the die to a die attach pad of a leadframe, bonding gold wires between bond pads on the die to corresponding leads on the leadframe and then encapsulating the die, die pad and wires in a resin material so that only the external leads of the leadframe project from the encapsulated package.
In recent years, electronic devices have been developed in which only the side of the leadframe to which the die is attached and the wire bonds are encapsulated leaving the other side of the leadframe free of resin. Such one-sided encapsulated electronic devices are known by a number of different names, including molded leaded packages (MLP), micro leadframe packages (MLF) and quad flat non-leaded packages (QFN).
However, one of the problems with one-sided encapsulated devices is to ensure that during the molding process, the resin covers only one side of the leadframe and does not flow onto the other side of the leadframe. If the resin flows onto the other side of the leadframe, this can result in a defect on that particular device resulting in the device having to be rejected.
One solution to minimising this problem has been to apply adhesive tape to the opposite side of the leadframe from the die side and to then attach the dies and perform the wire bonding, followed by the molding process. The theory behind the use of the tape is that the presence of the adhesive tape on the other side of the leadframe prevents seepage of the resin during molding onto the portion of the leadframe that will become the external leads, thereby reducing the number of devices that have to be rejected.
However, in practice, it is found that resin still flows onto the other side of the leadframe despite the presence of the adhesive tape. This is especially the case with matrix leadframes where the outside row of chips is particularly prone to resin seepage. Hence, it is common practice not to attach dies to the outside row of die attach portions as the probability of resin seepage on the outer row is relatively high.
Therefore, the seepage of resin to the other side of the leadframe reduces the yield of usable chips from a given leadframe.
In accordance with a first aspect of the present invention, a mold for molding a one-sided encapsulated electronic device comprises a first mold section defining a mold cavity, and a second mold section including a first recessed portion adapted to receive a layer of material attached to a leadframe, but not the leadframe.
In accordance with a second aspect of the present invention, a method of encapsulating a one-sided electronic device comprises forming a leadframe assembly by attaching a layer of material to one side of a leadframe, attaching a die to a die attach portion on the other side of the leadframe, bonding wires between contact portions on the die and contact portions on the leadframe, inserting the leadframe assembly into a mold with the die in a mold cavity in a first section of the mold and the leadframe being located on a second section of the mold with the layer of material, but not the leadframe, being located in a first recessed portion of the second section of the mold.
Typically, at least one dimension of the layer of material is smaller than the corresponding dimension of the leadframe. Preferably, the width of the layer of material is smaller than the width of the leadframe.
Preferably, the depth of the first recessed portion is greater than the thickness of the layer of material. Typically, the first recessed portion has a depth which is approximately 10% to 30% greater than the layer of material, preferably 15% to 30% greater, more preferably, 20% to 30% greater and most preferably, approximately 25% greater than the thickness of the material.
Typically, the leadframe is located in a second recessed portion of the second section of the mold, and the first recessed portion is located within the second recessed portion.
Preferably, the layer of material is an adhesive film that is attached to the leadframe by the adhesive. Typically, the adhesive film may be in the form of an adhesive tape. Preferably, the adhesive tape is a polyimide tape with a silicon adhesive, such as Nitto Denko TRM-6250 adhesive tape.
An example of a mold in accordance with the invention will now be described with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of a mold with a leadframe located in the mold after molding but before separation of the mold; and
FIG. 2 is a plan view of a matrix leadframe with adhesive tape attached and one of the matrix blocks molded.